Improved replication of vaccine seed strains in embryonated hens' eggs would lower the cost of manufacturing inactivated influenza vaccines and allow more doses to be manufactured. Higher-growing influenza A and B vaccine donor viruses will be developed that can be genetically reassorted with poorly-growing fresh human isolates to generate master vaccine seed strains that are antigenically appropriate and grow well in embryonated eggs. The influenza matrix (M) gene is important for high virus replication in eggs, and may function in virion assembly by interacting with viral glycoprotein cytoplasmic tails. To generate higher-growing viruses, the investigators will use random mutagenesis of M genes of strains that grow well in eggs (A/PR/8/34 and B/Lee/40). A cDNA library of mutant M genes will be screened using the yeast two-hybrid system, Shengqiang to detect mutant M genes that exhibit an altered interaction with the cytoplasmic tail of the influenza hemagglutinin protein. Altered M genes selected by the two-hybrid screen will be introduced into infectious influenza viruses using reverse genetics technology. The resulting viruses will be evaluated for enhanced replication in eggs and their ability to perform as high-growing vaccine donor strains.